Bubbling excess carbon (IV) oxide into calcium hydroxide solution results in the formaton of

Analyze the question: CO₂ is bubbled into limewater, Ca(OH)₂ solution, and the word excess is the key. You need to find the final product after all the CO₂ has reacted.

Concept: This reaction happens in two stages.

Stage 1: small amount of CO₂. The limewater turns milky because insoluble calcium carbonate forms.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Stage 2: excess CO₂. The extra CO₂ reacts with the CaCO₃ and water to form calcium hydrogen carbonate, which is soluble. The milky precipitate dissolves and the solution turns clear again.

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

Overall reaction with excess CO₂:

Ca(OH)₂ + 2CO₂ → Ca(HCO₃)₂

Checking the options:

• A. CaO: not formed. It is made by heating limestone.
• B. Ca(HCO₃)₂: formed with excess CO₂. ✔
• C. H₂CO₃: carbonic acid exists only in trace amounts, and it is not the main product with calcium hydroxide.
• D. CaCO₃: this forms only when CO₂ is limited, so it is the intermediate product and not the final one.

Answer: B. Ca(HCO₃)₂

Exam tip: Remember "milky, then clear." Limewater turns milky with a little CO₂ (CaCO₃) and clears again with excess CO₂ (Ca(HCO₃)₂). This is also why temporary hardness in water comes from Ca(HCO₃)₂. If a question says "excess," think hydrogen carbonate. If it says "small amount" or "limited," think carbonate.